Theoretical study on the relation between structural and optical properties in Si nanostructures

Two types of model silicon (Si) nanostructures in zero, one, and two dimensions are calculated. The model of the first type is the one adopted in most previous works, and its atomic configuration has high point-group symmetries; we refer to this model as a ‘high-symmetry’ model. On the other hand, our model of the second type is a ‘low-symmetry’ model, and its atomic configuration has no point-group symmetries. Since it is unlikely that realistic nanostructures have high symmetries, our model is more realistic for light-emitting Si. We calculate, in the tight-binding scheme, the electronic states, energy gaps, and the radiative recombination rates for these two models in zero, one, and two dimensions. We show that our ‘low-symmetry’ model yields a radiative recombination rate greater than that of a ‘high-symmetry’ model for zero- and one-dimensional systems. On the other hand, we show that the behavior of the radiative recombination rates for two-dimensional systems differs greatly from the case of zero- and one-dimensional systems. We show that, for two-dimensional systems, the radiative recombination rate is higher for the high-symmetry model. This result is interpreted in terms of a simple argument based on the phases of the wave functions for these systems.